Magnetic resonance imaging or MRI is a fast-growing diagnostic modality. The method was discovered towards the end of the 1970's and it provides similar slice images as X-ray tomography, but with improved contrast and resolution. The annual demand on the market is currently in the order of 1000 apparatus units. The most significant drawback of the method is probably its high costs. The price of a single installation is typically up to 1.5-2 million dollars or more. The most important reason for this is that the available pieces of apparatus are designed as general imaging eguipment and are capable of producing images of the entire human body. Therefore, their most expensive component, a magnet, becomes large in size and high in price.
There would also be a lot of interest in the market towards cheaper equipment that could be special equipment for just imaging some certain member of the body, such as the ankle, knee, wrist, maxillary joint, thorax etc. Thus, in the MRI-field there is a definite demand for inventions relating to better and cheaper imaging magnet systems.
The current MRI equipment employs three types of magnets: superconducting, resistive and permanent magnets. The superconducting ones produce the best result: a field generated thereby is at the same time powerful (&lt;1.5), stable and homogeneous in a large volume. However, this type of magnet is the most expensive one of them all. The permanent magnet is capable of producing a second strongest field (&lt;0.3-0.5T) but these magnets are very heavy and relatively expensive as well. The resistive magnets are the cheapest ones but produce the lowest field (&lt;0.1-0.2%) and often spend quite a lot of electricity and cooling water.
In his invention U.S. Pat. No. 4,906,931 R. Sepponen describes a means for lowering the price of an imaging magnet without compromising the image quality. The imaging is effected by means of two fields: The first step is the alignment or polarization of image-signal emitting protons (or other nuclei) with a powerful field Bp. This is followed by quickly changing the field into another Bo for effecting the actual imaging. A benefit gained by this method is that the imaging field Bo can be maintained quite low, for example 0.02-0.04 T, without actually losing any of the signal. A low field can be generated at a low cost, e.g. by means of a resistive magnet and, at the same time, it can be made sufficiently homogeneous and stable.
On the other hand, the polarizing field Bp can also be produced at a low cost, as its homogeneity and stability need not be particularly good. The Bp-field must be switched off rather quickly, appr. 0.1 s, as demonstrated by Macovski (A. Macovski et al.: Department of Electrical Engineering, Stanford University, Stanford, USA). It can be generated by using a magnet winding other than that used for producing Bo, whereby the generation thereof will be inexpensive since the Bp-magnet need not have particularly good homogeneity and stability.
An apparatus of the invention and its operation are illustrated in more detail in the accompanying drawing, in which